The subject matter of the present invention relates to a method and apparatus adapted to be disposed in a wellbore for measuring a resistivity of a conductive mud in the wellbore, and more particularly, to such method and apparatus which includes a tool string adapted to be disposed in the mud filled wellbore and an electrode array connected to the tool string, the electrode array including a first electrode located at the absolute bottom of the tool string and a second electrode, one of the electrodes emitting a current for propagation through the conductive mud between the first electrode and the second electrode, the current propagating from the first electrode, along a longitudinal axis of the tool string, to the second electrode and from the second electrode, along the longitudinal axis of the tool string to the first electrode.
Induction logging tools have been disclosed in the prior art. For example, U.S. Pat. No. 5,157,605 to Chandler et al discloses one such induction logging apparatus, the disclosure of which is incorporated by reference into this specification. In addition, U.S. Pat. No. 5,041,975 to Minerbo et al discloses a wellbore correction system for use in the induction logging apparatus of the Chandler et al patent, the disclosure in the Minerbo et al patent being incorporated by reference into this specification. The correction system in the Minerbo et al patent corrects an error inherent in a set of output signals which are produced from the well logging apparatus when the apparatus is disposed in a wellbore. The error represents the effect of the wellbore on the output signals from the well logging apparatus. In order to correct the effect of a mud-filled wellbore on the output signals from the well logging apparatus, it is often required to know the value of a resistivity of the mud in the wellbore, and the mud resistivity value must be known with good accuracy. The wellbore penetrates a formation, and the formation has its own resistivity value. Very often, there is a large difference between the resistivity of the mud in the wellbore and the formation resistivity. As the formation to mud resistivity contrast increases for a particular depth in the wellbore, certain undesired perturbations can be seen in the output signals produced from the well logging apparatus in the wellbore. As a result, correcting these perturbations, which exist in the output signals from the induction well logging apparatus, becomes mandatory, especially when the well logging apparatus is logging large wellbores.
This problem, relating to the undesired perturbations in the output signals, has been known in the prior art. To solve this problem, a special mud resistivity sub, known as an Auxiliary Measurement Sub (AMS sub), was developed. The AMS sub was located at the top of the induction well logging apparatus tool string. It provided measurements of the resistivity of the mud in the wellbore in addition to further measurements including temperature measurements and tool acceleration measurements. However, the AMS sub is plagued by two problems: measurement accuracy and cost.
With respect to the measurement accuracy problem, measuring the resistivity of the mud in a mud filled wellbore, without allowing the measurement to be affected by the resistivity of the formation penetrated by the wellbore or the position of the measuring tool within the wellbore, can be a very challenging problem, especially when the diameter of the wellbore ranges from 5 inches to 24 inches. One factor which may lead to an inaccurate measurement of the mud resistivity in a wellbore is the influence of the resistivity of the formation on the measurement. A resistivity probe, where 99% of its volumetric response comes from inside the wellbore, would make a 100% error during the estimation of mud resistivity in the wellbore when the formation penetrated by the wellbore is 100 times more resistive than the mud. As a result, the measurement probe must have an extremely shallow depth of investigation in the wellbore in order for the measurement probe to be immune to the formation resistivity. In order to reduce the adverse effect which the formation resistivity has on the accuracy of a mud resistivity measurement taken by the AMS sub, an outer wall of the AMS sub included a recess, and a set of electrodes were located within the recess. The electrodes of the AMS sub include current emitting, current receiving, and measurement electrodes. However, even though the set of electrodes were located within the recess in the outer wall of the AMS sub, the accuracy of the mud resistivity measurement taken by the AMS sub was still adversely affected by the differences between the mud resistivity in the wellbore and the formation resistivity penetrated by the wellbore.
With respect to the cost problem, since the AMS sub was located at the top of a tool string in the wellbore, it must provide means for electronically connecting a cable head with a remaining part of the tool string. As a result, an expensive multi conductor head (each head being a 31-pin head costing $10,000 dollars each) must be located at the top and at the bottom of the AMS sub thereby increasing the overall cost of the AMS sub.
Therefore, another method and apparatus is needed for taking mud resistivitymeasurements in a wellbore, especially when there is a significant difference between the resistivity of the mud in a wellbore and the resistivity of a formation penetrated by the wellbore.